1. The use of a signal on a control line of a circuit that changes the frequency of operation of the circuit. Examples of circuits include oscillators and filters where the frequency of the output signal can be changed by varying the voltage on a tuning-control input to the circuit.

2. The use of an electron beam in one circuit to change the operating frequency of a second circuit. The velocity, intensity, or configuration of the electron beam may be altered to provide the desired frequency change.